1. Field of the Invention
The present invention relates generally to gas turbine engine turbine flowpath seals and, more specifically, to attachment means for arcuate leaf seals therein.
2. Description of Related Art
A gas turbine engine includes a compressor for compressing air which is mixed with fuel and ignited in a combustor for generating hot combustion gases which flow downstream therefrom. The combustion gases flow through one or more turbine stages for extracting energy therefrom for powering the compressor and providing other useful work. A turbine stage includes a stationary turbine nozzle having a plurality of circumferentially spaced apart vanes extending radially between outer and inner bands which define a flowpath for channeling the combustion gases therethrough. Disposed downstream of the turbine nozzle is a turbine stage including a plurality of circumferentially spaced apart rotor blades extending radially outwardly from a rotor disk, and surrounded by an annular shroud which defines a portion of the radially outer flowpath for the combustion gases. The turbine nozzles and rotor shrouds are separately manufactured and assembled into position in the engine. Accordingly, gaps are necessarily provided therebetween for both assembly purposes as well as for accommodating differential thermal expansion and contraction during operation of the engine.
The gaps between these stationary stator components are suitably sealed for preventing leakage therethrough. In a typical high pressure turbine nozzle, a portion of the compressor air is bled therefrom and channeled through the nozzle vanes for cooling thereof. The use of bleed air reduces the overall efficiency of the engine and, therefore, is minimized whenever possible. The bleed air is at a relatively high pressure greater than the pressure of the combustion gases flowing through the turbine nozzle and, therefore, would leak into the exhaust flowpath without providing suitable seals between the stator components.
A particularly useful seal used to seal these gaps in the turbine flowpaths is a leaf seal. The leaf seals typically used in such applications are arcuate and disposed end to end around the circumference of the stator components which are segmented. For example, the radially outer band of the turbine nozzle includes axially spaced apart forward and aft rails. These rails extend radially outwardly, with the aft rail abutting a complementary surface on the adjoining shroud or shroud hanger for providing a primary friction seal therewith. The leaf seal provides a secondary seal at this junction and bridges a portion of the aft rail and the shroud hanger for example.
In order to assemble and mount the leaf seals to the aft rail, each leaf seal typically includes mounting holes at opposite circumferential ends thereof through which are mounted corresponding mounting pins. Corresponding springs such as leaf springs are also used at respective ones of the mounting pins for pre-loading the loosely supported leaf seals against the aft rail and the shroud hanger. During operation when air pressure is developed outboard of the outer band, the air pressure provides a substantial loading force against the leaf seal for improving its sealing effectiveness with the aft rail and the shroud hanger.
In order to support the leaf seals, leaf springs, and mounting pins, the outer band typically includes a plurality of circumferentially spaced apart, radially extending tabs spaced axially from the aft rail. A recess is formed between the tabs and the aft rail in which the leaf seal and leaf spring is disposed. The tabs include forward holes aligned with corresponding aft holes which extend into but not all the way though the aft rail. The mounting pins are inserted through holes in the tabs, leaf spring, leaf seal, and into the aft rail and then fixedly joined thereto by tack welding heads of the mounting pins to the corresponding tabs. In this way, the mounting pins are supported from both ends to the tabs and aft rail, and the leaf seals and leaf springs are trapped in the recess defined between the tabs and the aft rail. To accommodate relative movement between the nozzles and mating hardware, the leaf seals are designed to float about the mounting pins that are firmly attached to the nozzle segment. During engine operation, pressure differential between the cooling supply air and the flowpath air holds the seal against the mating hardware. Springs are used to provide positive contact at the sealing surface when pressure loading across the seal is low.
However, this mounting arrangement for the leaf seals is relatively complex and subject to damage during the assembly process in view of the relatively close quarters in this region. The leaf seals and springs are relatively small components, and the mounting tabs are therefore positioned relatively close to the aft rail which increases the difficulty of assembling and securing these components. The headed pins are located in holes drilled through mounting tabs and blind holes part way through the rail or flange. The aft rail and the tabs must be accurately machined to close tolerances, and the limited access provided due to their closeness increases the difficulty and cost of manufacturing the blind holes that extend part way through the rail or flange in an aftwardly direction. Following assembly, the mounting pin heads are tack welded to the tabs. Tack welding is complicated due to lack of space and the proximity of the thin sheet metal leaf seals and springs which are often damaged by inadvertent contact with the welding apparatus.
Manufacturing of the blind holes in the rail or flange is time consuming and expensive as each hole is required to be drilled through the tab but may not break through the aft nozzle flange due to leakage concerns (The pressure differential across this flange is very high). The current production process uses unique hook shaped EDM electrodes to generate these blind hole features. During nozzle segment repair, the mounting pins, springs and seals must be removed to facilitate removal and reapplication of the nozzle environmental coatings. Pin removal involves hand grinding of the tack welds which destroys the mounting pin head and can damage the cast tab. In addition, the blind holes must then be redrilled (EDM) to remove any coating build up due to the repair process.
A leaf seal mounting system is desirable for simplifying the manufacture and assembly thereof and eliminating tack welding of the mounting pins and drilling of the blind hole into the aft rail or flange.
A gas turbine engine arcuate segment has an arcuate band segment, at least one tab and an arcuate rail segment spaced axially apart from the tab and located along an axial end of the band segment. The tab and the arcuate rail segment extend radially away from the band segment and a space is located between the tab and the rail segment. The tab has a tab aperture entirely therethrough and the rail segment has a rail aperture entirely therethrough. The exemplary embodiment of the invention includes forward and aft cylindrical aperture sections, respectively, connected by a conical aperture section therebetween in the tab aperture and the forward cylindrical aperture section having a first diameter D1 that is smaller than second diameters D2 of the aft cylindrical aperture section and the rail aperture. A more particular embodiment of the invention is a leaf seal assembly and further includes the arcuate segment having an arcuate band segment with the tab and the arcuate rail segment spaced axially apart from the tab and located along an axial end of the band segment. The tab and the arcuate rail segment extend radially away from the band segment. A leaf seal and a spring are disposed within the space between the tab and the rail segment to bias the leaf seal against the aft rail. The spring has a spring aperture entirely therethrough, the leaf seal has a seal aperture entirely therethrough, and a headless pin is disposed in the apertures such that the leaf seal and the spring are radially restrained by the pin. The exemplary embodiment of the invention includes an axial restraining means for preventing axial movement of the pin out of the tab aperture. One embodiment of the axial restraining means includes an interference fit between the pin and the tab within the tab aperture. The pin has a shank connected to an axially forwardly located conical tapering pin section. The interference fit includes the conical tapering pin section abutting against the conical tapering aperture section. A securing means for axially securing the pin within the rail aperture includes, in the exemplary embodiment, a press fit of the pin in the rail aperture.
Another more particular embodiment of the invention is a gas turbine engine turbine nozzle segment having at least two circumferentially adjacent vanes that are joined together such as by brazing. Each vane has one or more hollow stator airfoils extending radially between radially inner and outer arcuate band panels, respectively. Circumferentially adjacent ones of the inner and outer band panels have corresponding inner and outer arcuate band segments of the nozzle segments. The aft rail segments are located at aft panel ends of the outer band panels, the tabs extend radially away from the outer arcuate band panels forming the spaces between the tabs and the aft rail segments. At least one of the tabs are on each of the outer arcuate band panels and the arcuate leaf seal is disposed within the spaces. The spring is disposed within each of the spaces between each of the tabs and the aft rail segments and the springs and leaf seal are operable to bias the leaf seal against the aft rail segments.
Yet another more particular embodiment of the invention is a gas turbine engine assembly having a first arcuate segment defining a flowpath of combustion gases and having an outwardly extending rail at one end thereof. A second arcuate segment is disposed coaxially with the first arcuate segment for defining a continuation of the flowpath and has a radially extending face adjoining the rail. The arcuate leaf seal bridges the rail and the face for sealing leakage therebetween of air outboard of the first segment. One embodiment of the gas turbine engine turbine assembly includes the arcuate gas turbine engine turbine nozzle segment axially adjacent to and forward of an arcuate shroud assembly portion. The portion includes circumferentially adjoining arcuate turbine shroud segment supported from a circumferentially adjoining shroud hanger. The shroud segment or the shroud hanger has a radially extending face. The arcuate leaf seal disposed within the spaces of the gas turbine engine turbine nozzle segment bridges the rail segment and the face for sealing leakage therebetween of air outboard of the nozzle segment.
The present invention simplifies and reduces the cost of manufacturing of the holes in the rail or flange by no longer using blind holes that are drilled through the tab but may not allowed to break through the rail or flange. The present invention eliminates the blind holes while still reducing or eliminating leakage through the holes. The invention also simplifies and reduces the cost of nozzle segment repair because it facilitates removal and reapplication of the nozzle environmental coatings. The invention also eliminates pin removal by hand grinding of the tack welds which destroys the mounting pin head and can damage the cast tab. The present invention provides a leaf seal mounting system that simplifies the manufacture and assembly thereof and eliminates tack welding of the mounting pins and drilling of the blind hole into the aft rail or flange.